Cerebellar damage disrupts movement coordination. Actions ranging from walking to reaching become inaccurate and difficult to control, resulting in poor functional movements. Therapeutic approaches for cerebellar motor disorders, or 'ataxia,' rely heavily on rehabilitation since there are no medications that systematically improve movement coordination. Rehabilitation is limited by the fact that cerebellar damage dramatically disrupts motor learning (i.e. the ability to learn movements through trial-and-error practice). Recently, we discovered that people with cerebellar damage can show motor learning, but only when the errors driving learning are small and gradual. It is also known that non-invasive stimulation of the brain can improve motor learning in healthy subjects. In this project, we will work to understand whether use of gradual learning strategies and non-invasive brain stimulation can improve learning of movement coordination of people with cerebellar damage. We will study both reaching and walking movements. In aim 1, we will test whether (a) the learning benefit associated with gradual learning is due to the small error size or due to the subject's lack of explicit awareness of error; (b) whether we can use gradual learning to teach people with cerebellar damage to correct their specific reaching deficits, and (c) whether gradual learning benefits extend to walking, which would imply a general brain mechanism for this type of learning. In aim 2 we will use transcranial magnetic stimulation (TMS) to test whether gradual motor learning is more associated with increased excitability in primary motor cortex, whereas learning from large and abrupt errors is more associated with increased excitability in the cerebellum. In aim 3, we will determine whether non-invasive, transcranial direct current stimulation (tDCS) of different brain regions can augment motor learning in people with cerebellar damage. In sum, an increased understanding of the mechanisms of motor learning could lead to important changes in rehabilitation treatments for people with ataxia from cerebellar damage. Our overarching goal is to provide a scientific foundation for devising new rehabilitation strategies.